Without trying to be a kill joy people completing the national three peaks are having a significant impact on the locals, particularly in the Lakes, so it's worth trying to minimise this if you're organising it yourself or checking organiser's policy if going with an organised group.
Have a read of this, old but the issues still persist
I think you need to build up slowly to running in the dark.
Too late for this year but next year during the summer months try running some of your shorter runs with your eyes closed to simulate darkness. Extend the distance you run with your eyes closed (only a bit a time mind; rememer the 10% rule!) and before you know it you'll have no trouble running in the dark at all.